1. Field of the Invention
The present invention relates to an image processing method, an image processing apparatus, and a color image generating system for converting monochrome data into color data.
2. Description of the Related Art
Color documents are often printed in monochrome. Such instances, more often than not, involve a case where an available printer is a monochrome printer or a case where a monochrome printing mode is selected in a color printer to reduce a printing cost. In the monochrome printing, although colors of the document are not reproduced, a grey scale image having shades of gray according to the original colors can be printed. This makes it possible to distinguish, in a certain degree, differences in the original colors from the finished print.
On one hand, color document is printed in monochrome in this way. On the other hand, there are demands for colorization of the monochrome documents. Specifically, there is a case in which a monochrome printed matter or monochrome data is only available at hand, but a color document based thereon is demanded. The colorization in this context does not mean restoration of original colors of an image from which colors were converted into a monochrome image but the colorization according to the shades of gray (lightness). For example, in a case where a document based on which colorization is demanded is a monochrome image converted from original colors, the monochrome document may have portions whose identification from each other is made difficult by the monochromatic conversion because two distinct original colors have been converted into densities (lightness) extremely close to each other. In such a case, colorization makes it possible to colorize again, using colors but may be different from the original, the portions used to be separated by different colors. As a result, such portions can be easily identified from each other.
In the conversion from the monochrome data to the color data, each pixel is assigned with a hue in accordance with its gray level. According to a related art disclosed in JP-H10-324025A, a monochrome image is converted into a color image using a color conversion code that is prepared in advance. In preparing the color conversion code, the lightness is changed in proportion to a gray level of the monochrome image, and at the same time the color saturation and the hue are also changed in proportion to the gray level so that a change in color corresponding to individual steps between levels of gray is made uniform and a change in color perception is made smooth. The colors represented by the color conversion code thus obtained correspond to a helically crawling path on a surface of color space starting from “white” ending at “black” in the color space formed of two cone shapes that are put together at their individual bottom faces as illustrated in FIGS. 4 and 6 of JP-H10-324025A. According to this related art, since the gray levels of the monochrome image are reflected in the lightness of colors, it is possible to obtain a colorized document having a natural contrast.
However, according to the colorization in the related art, colorization for one or both of a halftone portion in the vicinity of the palest (lightest) white and a halftone portion in the vicinity of the densest (darkest) black is not conspicuous. This may result in observation with a reduced number of colors.